Variable resistance device



April 13, 1965 w. J. H. THOELE 3,178,664

Y VARIABLE RESISTANCE DEVICE Filed Jan. 15, 1962 2 Sheets-Sheet 1 es L64 as &\ 62 6 FIG. 14 4 5 6 35 fl /k bw X q l 50 /49 m\\\\ 31: -u

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INVENTOR.

B WILLIAM J.H. THOELE ATTORNEY April 13, 1965 w. J. H. THOELE INVENTOR.

.BY WILLIAM J.H. THOELE ATTORNEY United States Patent 3,178,664 VARIABLE RESISTANCE DEVICE William J. H. Thoele, Santa Ana, Califi, assignor to Beckman Instruments, Inc., a corporation of California Filed Jan. 15, 1962, Ser. No. 166,199 17 Claims. (Cl. 338-183) This invention relates to variable resistance devices and, more particularly, to a variable resistance device having an improved means for converting the rotary motion of the adjustment shaft into rectilinear motion of the movable electric contact. The present invention finds particular utility in the class of potentiometers known in the art as trimming potentiometers.

In contemporary rectilinear trimmer otentiometers, it is conventional to incorporate a rotatable lead screw which cooperates with a threaded travel block. Rotary motion of the lead screw causes translatory motion of the travel block and the movable electrical contact associated therewith relative to the potentiometer housing. In potentiometers of this type, however, axial motion of the lead screw may also impart translatory motion to the travel block and thus effect an undesirable change in the resistance or voltage between the movable electrical contact and the end terminals. Similarly, the effect of dif ferential linear thermal coeflicients of expansion between the lead screw and housing may cause a change in contact position with a detrimental effect upon the stability of the potentiometer setting.

It is an object of this invention to provide a rotatably operated rectilinear resistance device which does not incorporate a threaded lead screw.

It is another object of this invention to provide a variable resistance device in which the contact is not subject to unwanted motion caused by differential linear thermal coefficients of expansion of the component parts of the variable resistance device.

It is still another object of this invention to provide a variable resistance device in which the shaft may be moved axially without imparting a corresponding movement to the movable electrical contact.

Another object of this invention is to provide a variable resistance device in which movement of a single shaft in an axial direction causes a first resistance to change independent of a second resistance and in which rotary motion of the same shaft causes a change of the second resistance independent of the first.

Other and further objects, features, and advantages of the invention will become apparent as the description proceeds.

Briefly, in accordance with a preferred form of the present invention, there is provided a variable resistance incorporating a longitudinal resistance element and an electrical contact element adapted for movable engagement with the resistance element. This contact is part of a contact assembly which includes a contact block and worm gear, the worm being mounted by the contact block for rotation about its spiral axis but being prevented by said contact block from axial motion with respect thereto. A control shaft is rotatably mounted by the potentiometer housing parallel to the resistance element. This shaft extends completely through the worm gear concentric with its axis of rotation, these elements being so formed as to allow longitudinal movement of the worm relative to the shaft but preventing rotation of one relative to the other. The contact block is also constructed so as to expose a portion of the threads of the worm gear. These threads engage a rack gear integral with the variable resistance housing. Thus, as the control shaft is rotated, the worm gear travels down the rack gear, moving with it the travel block and the attached electrical contact.

3,178,654 Patented Apr. 13, 1965 Additional details of the variable resistance include means which may be constructed in the conventional manner for making electrical contact between the outside of the housing and the electrical contact carried by the movable contact block and for making electrical connection with one or both ends of the resistance element.

A more thorough understanding of the invention may be obtained by study of the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-section of a typical trimmer potentiometer constructed according to the present invention;

FIG. 2 is a perspective view of the potentiometer housing;

FIG. 3 is a perspective view of the potentiometer housing cover plate;

FIG. 4 is a cross-sectional view of the structure of FIG. 1 taken along the line 4-4;

FIG. 5 is a cross-sectional view of the structure of FIG. 4 taken along the line S-5;

FIG. 6 is a crosssectional view of the structure of FIG. 1 taken along the line 6--6;

FIG. 7 is a perspective view of the potentiometer resistance element base;

FIG. 8 is a perspective view of the potentiometer movable electrical contact;

FIG. 9 is a perspective view of the potentiometer travel block;

FIG. 10 illustrates a typical staple for joining the movable electrical contact to the travel block;

FIG. 11 is a perspective view of the potentiometer control shaft and O-ring seal;

FIG. 12 is a perspective view of the worm gear employed in the movable contact assembly;

FIG. 13a, 13b and illustrate other embodiments of the control shaft; and

FIG. 14 illustrates ganged rectilinear trimmer potentiometers independently operable by a single control shaft.

Referring now to FIG. 1, there is shown a trimmer potentiometer constructed according to the present invention having a housing 10, a rectilinear resistance element 11 mounted within the housing, a movable electrical contact 12, a travel block 13 to which the electrical contact 12 is fixedly mounted, and an actuation mechanism 14. As described in detail below, this assembly includes control shaft 15 which when rotated causes a corresponding translatory motion of the travel block 13.

It will be understood that the trimmer potentiometer shown in the figures is considerably enlarged for illustrative purposes. Representative outside dimensions of potentiometers constructed in accordance with this invention are 1% x 3 x A".

Housing 19 shown in detail in FIGS. 2 and 3 is preferably constructed according to the teachings of the copending application entitled Improved Variable Resistor, filed on even date herewith by Jack E. Langenbach et al. and assigned to Beckman Instruments, Inc., assignee of the present invention. As shown in FIGS. 2 and 3, this preferred construction comprises a six-sided generally rectangular housing 10 preferably molded from a plastic material or a refractory material, or die-cast from a metal or metallic alloy, the particular metal and mode of construction depending upon the environmental strength and temperature conditions in which the potentiometer will be used.

The housing cover plate 16 (FIG. 3) is preferably molded from the material of the housing 10 and includes integral therewith a rear half-bearing 1S and a shaft keeper and front half-bearing 17. The housing cover plate is shown attached to the housing in FIGS. 4 through 6, these members normally being joined by eyeleting through the two holes provided in the housing and the housing cover plate. Of course, other methods of attaching the cover plate to the housing may be employed, such as rivets, screws, or adhesives.

The housing further includes a rectangular cavity 19 in which is located the resistance element base 21 (FIG. 7). In order to insure that the base element is mounted firmly with respect to the housing 16, it is generally cemented thereto by a suitable epoxy adhesive. This base may be constructed of any suitable material although ordinarily it will be of a material having a high dielectric resistance and adapted to undergo the environmental conditions to which the resistance element will be exposed. For example, the base element 2% may be of a ceramic material such as steatite upon which is formed a cermet resistance element according to the teachings of US. Patents Nos. 2,950,995, of Tom Place, Sr., et al., and 2,950,996, of Tom Place, Sr., et al., both of which are assigned to Beckman Instruments, Inc. It will of course be apparent to those skilled in the art that other forms of resistance elements may be employed in the variable resistor shown, examples being wirewound resistance elements, conductive plastic resistance elements and deposited film type resistance elements. The base 20 further includes through openings 21, 22, and 23 which receive electrically conductive terminals 24, as described below.

The electrical contact element 12 and the movable contact block 13 are respectively shown in detail in FIGS. 8 and 9. The electrical contact of FIG. 8 is of a customary design in which a sheet of conductive material is formed in a generally V-shape with the lower leg having a plurality of slots 25 therebyproviding a plurality of split contact elements 26, the ends of which are arcuately shaped. Contact element 12 is retained to the underside of travel block 13 by staple 27 which engages the underside of upper leg 28 of contact 12 and passes through holes 29, 30 of the block 13 after which the ends of the staple are bent over thereby fixedly mounting the contact element 12 with respect to the contact block 13. Contact block 13 further forms a portion of the actuation assembly 14 described below.

It will be understood that the particular movable electrical contact 12 shown is by way of illustration only, other types of contacts known in the art may be preferred in certain applications depending upon the type of resistance element utilized. For example, in many applications it may be desirable to use the conductive plastic contact described in the application of Kenneth F. Miller, et al., entitled Variable Resistor Contact, filed December 11, 1961, Serial No. 158,318, and assigned to Beckman Instruments, Inc. Likewise, other means may be used to attach the contact 12 to the block 13, e.g., suitable cements may be used to adhere the contact to the block or a portion of the contact 12 may be embedded in the block 13.

Travel block 13 with the electrical contact 12 attached thereto are shown mounted within the potentiometer housing 10 in FIGS. 1 and 5. As shown in FIG. 5, a plurality of the split contact elements 26 engage the rectilinear resistance element 11 and a lesser number engage a conductive rectilinear strip 31 mounted on the resistance base 20 in spaced, parallel relationship to the resistance element. In a manner well known in the potentiometer and variable resistor arts, the engagement of the movable electrical contact 13 with the conductor strip 31 provides the required connection between a stationary electrical lead and the movable electrical con tact. As shown in FIG. 1, conductive terminals 24, preferably metallic pins, extend through respective openings 21, 22 and 23 provided in the resistance base 20 and engage the ends of resistance element 11 and the conductive strip 31. These pins are also respectively connected to electrical leads 32, 33 and 34 which extend through an appropriate aperture 41 in the rearward portion of the housing 111. In an alternative arrangement, contact pins (not shown) adapted for use with printed circuitry may extend through the bottom of the housing It).

The actuation assembly 14 comprises travel block 13, the control shaft 15 shown in detail in FIG. 11, the Worm gear 35 shown in detail in FIG. 12, and the rack gear 36 integrally formed on the top inside surface of the housing 11 as shown in FIGS. 1 and 2. The teeth of this rack are formed with the same pitch as the threads of the worm gear 35. As shown in FIGS. 1 and 5, Worm gear 35 is retained in the semi-circular trough 37 formed in the top surface of the travel block 13 (FIG. 9). This trough is partially closed at each end and the length of the worm is dimensioned to be a line to line fit with the respective ends thereby preventing longitudinal movement of the worm gear with respect to the travel block 13. The worm 35 is however so dimensioned that its diameter is slightly smaller than the diameter of the semi-circular trough 37 thereby allowing free rotation of the worm about its spiral axis with respect to the travel block 13. As described in the co-pending application entitled Improved Variable Resistor, supra, ribs 49 and 5t) respectively formed within the housing It and housing cover plate 15 prevent rotation of the travel block with respect to the potentiometer housing (FIG. 5).

Worm gear 35 is mounted on and rotates in unison with the control shaft 15. For this purpose, the worm gear 35 has a longitudinal hole 38 including a pair of projecting keys 39 which mate with corresponding longitudinal slots or key ways 40 in the control shaft 15. Optionally, the shaft could be of practically any noncircular cross-section, e.g. the square shaft 41 shown in FIG. 13a, the triangular shaft 42 shown in FIG. 13b, and even an oval shaped shaft 43 as shown in FIG. 130, each of which require only that the longitudinal hole in the worm gear match the geometry of the shaft. This construction, while permitting translatory movement of the worm relative to the shaft prevents rotational movement between these members.

As is described in more detail in co-pending application entitled Improved Variable Resistor, supra, the shaft 15 is mounted in half-bearings formed in the housing 10 and in the housing lid 16. Moreover, the front half-bearing and shaft keeper 17 on the housing cover plate 16 extends into the groove 45 formed in the shaft 15 thereby retaining the shaft against longitudinal move ment with respect to the housing in those applications requiring this type of construction. As also described in this co-pending application, O-ring 46 restrains the control shaft against unwanted movement and conditions of shock and vibration while not exceeding the maximum operating torque allowed by military specifications, and seals the shaft opening, thus preventing the ingress of undersirable environmental materials.

The exposed threads of the worm gear 35 mate with like pitched teeth of the rack gear 36 in the assembled resistance element, as shown in FIGS. 1 and 5. Accordingly, as the control shaft 15 is rotated about the bearings defined by the housing 10 and the housing cover plate, the worm gear 35 travels down the rack gear 3d due to the engagement of the worm threads with the rack teeth. The rectilinear movement of the Worm causes it to exert a force on the closed end of the semicircular trough 37 of the travel block 13, thereby imparting a corresponding rectilinear motion to the travel block and the movable electrical contact 12.

In rectilinear variable resistances of the trimmer potentiometer type, it is not customary to provide a mechanical means for indicating the contact position relative to the resistance element. It is therefore desirable to provide a means of preventing damage to the component parts of the potentiometer if rotation of the shaft is continued after the contact travel block has reached the end of its excursion. Accordingly worm gear 35 is preferably formed from a resilient material with a low coeflicient of friction and a good cold memory, such as polytetrafluoroethylene, or monochlorotrifiuoroethylene or nylon. By referring to FIG. 5, it may be noted that less than 90 degrees of the worm thread is engaged by the rack gear 36. When the contact block 13 reaches the end of an excursion and rotation of the control shaft 15 is continued, the resilient nature of the material of the Worm permits the threads to be deformed and to jump or trip out of engagement with the rack, and with still further rotation of the shaft, re-engage the rack without damage to the component parts. The additional shaft torque required to deform the worm threads can be detected during manual operation of the shaft and thus signify to the operator that the excursion of the contact carrier has been attained. The amount of torque necessary to rotate the shaft after the travel block has reached the end of its excursion may be varied during the construction of the variable resistance by increasing or decreasing the number of the worm gear threads which will be out of engagement with the rack gear 36 by respectively decreasing or increasing the number of rack gear threads 36 at its respective ends in the areas 41 and 48 shown in FIG. 1. Thus, the blank spaces 47 and 48 are sufiiciently long so that if all but one thread of the worm enters in this area, a relative ly low escapement torque will be provided whereas if the rack is lengthened so that all but one or two of the worm gear threads are in engagement therewith, the escapement torque is correspondingly increased.

In FIG. 14 is illustrated a pair of ganged rectilinear potentiometers 55, 56 independently controlled by a common control shaft 57. Thus, linear actuation of control shaft 57 varies the output of potentiometer 55 whereas rotation of control shaft 57 varies the output of potentiometer 56. This operation is provided by con structing shaft 57 from a substantially uniform length of cylindrical stock with a groove or grooves provided therein for rotating worm gear 58 of potentiometer 56. This Worm gear 58 cooperates with a rack gear 59 for translating the travel block 60 and movable electrical contact 61 with respect to the resistance element and commutator affixed to the resistance element base 62. In the potentiometer 55, however, the control shaft 57 is affixed to a cylindrical member 63 which rotates within the semi-circular trough of the travel block 64. Thus, as the shaft 57 is linearly actuated, the member 63 engages the travel block 64 to translate the movable contact 65 with respect to the resistance element and commutator affixed to the base 66. Rotation of the shaft 57 merely rotates member 63 in the travel block 64 and has no effect upon the position of movable contact 65, while linear actuation of shaft 57 merely translates same with respect to the worm gear 58 whose threads remain engaged in the rack 59. Accordingly, the translation of shaft 57 has no effect upon the position of movable contact 61 with respect to the resistance element and commutator aflixed to the base 62. Although the variable resistance device of FIG. 14 is shown constructed from a pair of ganged potentiometers, it will be apparent that the resistance element bases and actuation mechanisms could be embodied within a single unitary housing without departing from the present invention.

Variable resistance elements constructed in accordance with this invention have several advantages over those available heretofore. Thus, it will be apparent from the foregoing description that if any longitudinal movement of the control shaft 15 with respect to the housing is permitted by construction tolerances, the setting of the electrical contact 12 with respect to the resistance element 11 will not be affected since the shaft will merely slide with respect to the worm gear 35 and the control block 13. Only a rotational movement of the control shaft with respect to the housing causes a movement of the travel block 13. Another advantage g of the present invention is the elimination of a threaded control shaft. As described hereinabove, this invention can utilize a shaft of practically any non-circular configuration. Still another advantage of this invention is that each of the variable resistance elements are easily constructed, the rack gear 36 for example may be molded at the same time as the housing 10 is molded. The worm gear 35 and travel block 13 may likewise be molded components, even in the very small sizes required for trimming potentiometers.

Although an exemplary embodiment of the invention has been disclosed herein for purposes of illustration, it will be understood that various changes, modifications and substitutions may be incorporated in such embodiment without departing from the spirit of the invention as defined by the claims which follow.

I claim:

1. A variable resistor comprising:

a housing;

a resistance element mounted within said housing;

a movable electrical contact engaging said resistance element;

and means for moving said electrical contact with respect to said resistance element including:

a rack gear fixedly mounted with respect to said housing,

a control shaft rotatably mounted with respect to said housing, a portion of said shaft having a non-circular cross-section,

a worm gear having an opening through its spiral axis slidably mating said non-circular port-ion of said control shaft, said worm being mounted upon said control shaft with a portion of its threads in engagement with said rack,

and means for rotatably mounting said worm gear with respect to said movable contact while preventing axial movement of said worm gear With respect to said electrical contact.

2. The variable resistor defined in claim 1 wherein:

said non-circular portion of said control shaft includes a longitudinal slot which engages a projecting key formed within the opening of said worm gear. 3. The variable resistor defined in claim 1 wherein: said non-circular portion of said control shaft includes a pair of oppositely disposed longitudinal slots, and said worm gear includes a pair of projecting keys within the opening thereof which engage said slots.

4. The variable resistor defined in claim 1 wherein:

said non-circular portion of said control shaft and the opening through said Worm gear have substantially rectangular cross-sections.

5. The variable resistor defined in claim 1 wherein:

said non-circular portion of said control shaft and the opening through said worm gear have substantially triangular cross-sections.

6. The variable resistor defined in claim 1 wherein:

said non-circular portion of said control shaft and the opening through said worm gear have substantially oval cross-sections.

7. A variable resist-or comprising:

a housing;

a rectilinear resistance element mounted within said housing;

a movable electrical contact engaging said resistance element;

and means for providing translatory movement of said electrical contact with respect to said resistance element including:

a rack gear fixedly mounted with respect to said housing,

a control shaft rotatably mounted with respect to said housing, said shaft having a non-circular cross-section throughout a greater portion of its length within said housing,

a worm gear having an opening therethrough mating with said non-circular portion of said control shaft, said worm gear being slidably mounted upon said control shaft and rotating in unison therewith about its spiral axis, the threads of said worm engaging the teeth of said rack gear,

and means for rotatably mounting said worm gear with respect to said movable contact while preventing axial movement of said worm gear with respect to said electrical contact.

=8. The variable resistor defined in claim 7 wherein:

said worm gear is formed from a resilient material with a low coefiicient of friction and a good cold memory so that when said movable contact reaches the end of an excursion and rotation of the control shaft is continued, the resilient worm construction permits the Worm threads to be temporarily deformed and temporarily disengaged from the teeth of said rack gear.

9. The variable resistor defined in claim 8 wherein:

said worm gear is constructed of polytetrafluoroethylene.

.10. The variable resistor defined in claim 8 wherein:

said worm gear is constructed of monochlorotrifluoroethylene.

1 1. The variable resistor defined in claim 8 wherein:

said worm gear is constructed of nylon:

12. The variable resistor defined in claim 7 wherein:

said rack gear is somewhat shorter than the allowable travel of said movable contact gear so that a portion of the worm gear threads are out of engagement with the rack gear at the end of an excursion of said movable contact.

13. The variable resistor defined in claim 7 comprising:

means for varying the amount of torque necessary to rotate the control shaft after the movable contact has reached the end of its excursion.

14. The variable resistor defined in claim 13 wherein:

the torque is increased by increasing the number of rack gear teeth engaged by the worm gear at the end of the excursion of said movable contact.

15. A variable resistor comprising:

a housing;

a rectilinear resistance element mounted within said housing;

a spaced rectilinear conductive element mounted within said housing parallel to said resistance element;

a movable electrical contact engaging both said resistance element and said conductive element;

and means for moving said movable electrical contact relative to said resistance element and said conductive element comprising:

a travel block mounting said movable electrical contact to the underside thereof, said travel block having a semi-circular trough in its upper surface,

a worm gear located in said trough so as to freely rotate with respect thereto While being restrained from axial motion relative to said travel block,

a control shaft rotatably mounted by said housing and passing completely through the spiral axis of said worm gear, said worm gear being adapted for axial movement relative said control shaft while being constrained to rotate in unison therewith,

a rack gear having plurality of teeth with the same pitch as the threads of said worm gear fixedly mounted with respect to said housing so as to engage the worm gear threads whereby rotation of said control shaft causes the and worm gear to travel along the rack gear moving with it the travel block and the movable electrical contact afiixed thereto.

16. A variable resistor comprising:

a tnolded housing, said housing having an inner cavity in which is located a rack gear molded integral with a wall of said cavity;

spaced rectilinear resistance and conductive elements fixedly mounted within said cavity opposite said molded rack gear;

a worm gear whose threads have the same pitch as said rack gear;

a control shaft passing through the spiral axis of said Worm gear, said shaft being rotatably mounted by said housing parallel to said resistance and conductive elements so as to engage the worm gear with said molded rack gear;

means integral with said worm gear and said control shaft for allowing translatory movement of said worm gear with respect to said control shaft while preventing rotation of said worm gear about its spiral axis with respect to said control shaft;

a travel block engaged by said worm gear and movable in an axial direction as said worm gear travels along said rack gear upon rotation of said control shaft;

and a movable electrical contact afiixed to said travel block and an electrical engagement with both said resistance and conductive elements.

17. A variable resistor comprising:

a housing;

a resistance element mounted within said housing;

an electrical contact assembly movably mounted relative to said resistance element including:

an electric con-tact engaging said resistance element,

a contact carrier mounting said electric contact,

and a worm gear mounted by said contact carrier for rotation relative thereto about its axis but being restrained thereby from longitudinal motion relative said contact carrier;

a shaft rotatably mounted by said housing parallel to said resistance element, said shaft passing through an opening of said worm gear concentric with its axis of rotation;

means on said worm gear and said shaft for preventing one from rotating with respect to the other while permitting longitudinal motion therebetween;

and a rack gear mounted within said housing and engaging exposed threads of said worm gear.

References Cited by the Examiner UNITED STATES PATENTS 416,632 12/89 Leber 3 38-74 530,4-65 12/94 Van Vleck 338-126 789,456 5/05 Rupley 338-182 884,458 4/08 Brice 338-74 1,051,749 1/13 Nispel 338-72 1,506,070 8/24 Leslie 338-116 1,684,013 9/28 De Pue 338-141 2,178,314 10/39 Saxe 235-61 2,197,312 4/40 Nelson 325-395 2,273,760 2/42 Nelson 338- 2,516,376 7/50 Field et a l. 343-112 2,704,800 3/55 Graybeal et a1. 338- 2,711,463 6/55 Goeppinger 338-323 2,857,497 10/58 tBourns et al. 338-183 2,860,216 1 1/58 Hubbard et al. 338-180 2,926,324 2/60 Barden et a1. 338-183 2,953,763 9/60 Eourns et al. 338-183 2,980,876 4/61 Larson 338-183 RICHARD M. WOOD, Primary Examiner. 

1. A VARIABLE RESISTOIR COMPRISING: A HOUSING; A RESISTANCE ELEMENT MOUNTED WITHIN SAID HOUSING; A MOVABLE ELECTRICAL CONTACT ENGAGING SAID RESISTANCE ELEMENT; AND MEANS FOR MOVING SAID ELECTRICAL CONTACT WITH RESPECT TO SAID RESISTANCE ELEMENMT INCLUDING; A RACK GEAR FIXEDLY MOUNTED WITH RESPECT TO SAID HOUSING, A CONTROL SHAFT ROTATABLY MOUNTED WITH RESPECT TO SAID HOUSING, A PORTION OF SAID SHAFT HAVING A NON-CIRCULAR CROSS-SECTION, A WORM GEAR HAVING AN OPENING THROUGH ITS SPIRAL AXIS SLIDABLY MATING SAID NON-CIRCULAR PORTION OF SAID CONTROL SHAFT, SAID WORM BEING MOUNTED UPON SAID CONTROL SHAFT WITH A PORTION OF ITS THREADS IN ENGAGEMENT WITH SAID RACK, AND MEANS FOR ROTATABLY MOUNTING SAID WORM GEAR WITH RESPECT TO SAID MOVABLE CONTACT WHILE PREVENTING AXIAL MOVEMENT OF SAID WORM GEAR WITH RESPECT TO SAID ELECTRICAL CONTACT. 